The collective terms galactomannans and polygalactomannans include all polysaccharides that are constructed of mannose and galactose building blocks and also have other sugar building blocks to a subordinate extent. Depending on the source, there is a relatively large number of galactomannans. They originate primarily from the endosperm portions of seeds of various legumes, such as guar, carob, tara, Cassia occidentalis, flame tree [literal], Sesbania cannabia, Sesbania aculeata, Sesbania egyptiaca and honey beans [literal]. Polygalactomannan derivatives that can be produced by the method according to the invention are hydroxyalkyl ethers of polygalactomannans, e.g. hydroxypropyl ethers and hydroxyethyl ethers, carboxyalkyl ethers of polygalactomannans, e.g. carboxymethyl ethers and carboxyethyl ethers, mixed carboxyalkyl- and hydroxyalkyl-substituted polygalactomannans, depolymerized polygalactomannans and quaternary ammonium ethers of polygalactomannans.
Polygalactomannan flours and flours of polygalactomannan derivatives, especially when the individual flour particles are very fine (particle diameters less than 100.mu.), cause considerable problems when dissolved in water as a result of excessive lumping. These problems have their origin in the fact that upon contact with water, a gel layer forms very rapidly on the surface of the individual particles or aggregates of particles and prevents the water from further penetration and thus leads to the formation of lumps which are difficult and time-consuming to dissolve further.
Various possibilities are currently known for eliminating these problems. For example, it is possible by means of vigorous stirring to prevent lumping of the flour when it is added to water or to reduce or destroy already formed lumps. However, this requires additional, expensive equipment, such as high-powered stirrers or special injection systems.
Another possibility consists of forming a suspension of the flour in a medium that does not have a hydrating effect, e.g. mineral oils or alcohols, and then adding these to the water. However, this requires additional organic solvents whose presence in the prepared galactomannan solution is often undesirable.
It is also known that borate ions can be mixed into the polygalactomannans in the required concentration as a dispersing agent. The minimum concentrations of borate ions, based on borax (Na.sub.2 B.sub.4 O.sub.7.10H.sub.2 O), that are necessary to prevent hydration of the flours are dependent on the pH. For example, for 1% guar flour in water at pH 10-10.5, only 0.25-0.5% borax (based on the weight of the guar flour) is required, and at pH 7.5-8, 1.5-2% borax is required (R. Whistler, Industrial Gums (1959)).
The addition of borax to the galactomannans can be performed in various ways. Besides dry mixing of borax to the ground galactomannan, it is also possible to incorporate the borate ions into the flours using an alkaline aqueous/methanolic phase.
It is also possible to incorporate the borax into the galactomannan before grinding the endosperm, e.g. as described in German Patent [Auslegeschrift] No. 23 12 351, according to which, the borax concentration comes to 0.1 to 0.1 to 0.5 wt% of the polygalactomannan.
When macromolecular compounds that have been pretreated in this way are added to water, the borate ions prevent the individual polymer particles from sticking together. The effect of the treatment of macromolecular compounds with borate ions is presumably due to a slight crosslinking that occurs on the surface of the polymer particles and, depending on the type of treatment, is completely or almost completely reversible and which results in a retarded swelling, sinking and distribution of the fine particles of the substance in the water and thus a lump-free solution.
The complex-forming reaction of borate ions with polygalactomannans is reversible, a reduction of the pH value below 7 permits normal swelling of the polymer particles for thickening. A shift of the pH back into the alkaline range can, due to the crosslinking reaction of the borate ions with polygalactomannan compounds, result in a viscosity increase up to the crosslinking.
Although it is naturally desirable to use a minimum amount of borax, according to German Patent [Auslegeschrift] No. 23 12 351, it must be regarded as inadvisable to fall below the limit of 0.1 wt% borax, based on the polygalactomannan, which is given in that patent as the lowest borax concentration (according to the single example, it is 0.2 wt% borax).